Scottie has pretty much hit the nail on the head.
The charter flight is operated by Monarch airlines but is chartered to either one or more than one tour companies. This is much like you chartering a flight or a bus or even a taxi. You would have something to say if the operator said "oh by the way I have decided to pick up a few other fares to fill the plane/ bus/ car. In other words the contract exists only between the charterer and the operator.
A scheduled service ( plane or train or bus ) plies a route and offers direct contracts ( tickets) between individual customers and the operator.
It is not impossible that a block of seats on a scheduled flight may be chartered under separate contract or even that a block of seats on a charter flight be be avilable for individual sale but it is not usual in the circumstances you describe.
These different modes of operating ( scheduled and charter ) not only operate under different contractual arrangements but usually also operate under different regulatory rules. An airline may be allowed to operate a certain number of flights on a particular route and sell a certain number of seats. Charter arrangements are often not so restrictive under competition rules but would almost certainly preclude the sale of individual seats on an open market arrangement, if not otherwise under some form of tour or inclusive holiday agreement.
I am sure that if exceptional compassionate circumstances existed the airline and indeed the tour operator may well have bent over backwards to assist. This doesn't appear to be the case here, but I hope this explanation is of some assistance